Here is the "Electrician's Mate 3 - Navy Training Courses"
(NAVPERS 10548) in its entirety (or will be eventually). It should provide one of
the Internet's best resources for people seeking a basic electricity course - complete
with examples worked out. See
See Table of Contents.
¶ U.S. GOVERNMENT PRINTING OFFICE;
Navy searchlights are classified as to the size and source of light. The 36-inch and 24-inch searchlights use
high intensity carbon arcs as the source of light, while the 12-inch searchlights use an incandescent lamp, usually
a 1,000-watt lamp. The size of these searchlights is determined by the diameter of the parabolic reflector.
Iris Mechanism for 24" U.S. Navy Searchlight (photo by Tim P.)
"Kirt, I just found your website while looking
for an explanation of the purpose of a device that a friend of my owns. It is an iris shutter for a 24" naval searchlight.
Your site and description of the searchlight helped to figure out the mystery of this thing!"
"I was at a friends
party new years eve and he dragged the thing out of a closet and started showing it off to those of us that were
interested. It weighs about 50 lbs. I would say, it is made out of cast aluminum and the glass dome is surprisingly
thin. Tempered no doubt, and it says Bausch & Lomb right on the glass. My friend bought it from an antique
USES OF SEARCHLIGHTS
The 36-inch light is used primarily for fire control. Usually they are equipped with remote control systems so
they can be trained and elevated from the gun fire control director. Most are equipped with an iris shutter and may
be operated either manually or by an automatic motor. The 24-inch searchlight is a general purpose light and although
it may be used for fire control purposes, generally it is used for long range signaling. These searchlights are not
equipped with remote control for train and elevation. The light sometimes is equipped with the remote control signal
key. This lamp usually has both the iris and vane type shutter.
The 12-inch searchlight is used primarily
for short range signaling. These lights are not equipped with remote control equipment, and have a manually controlled
vane type shutter.
REQUIREMENTS OF A SEARCHLIGHT
A Navy searchlight must have a narrow, pencil-like light beam. The light must be non-flickering and of a 'bluish
white color. It must operate properly from any position, under all weather conditions, and give trouble-free operation
over a long period of time. The train and elevation characteristics of most lights are: train through 360°, elevation
of 110° to 120°, and depression of 30° to 40°.
THEORY OF CARBON ARC SEARCHLIGHT
When an electric current at a moderate voltage is passed through two carbon rods, considerable heat is developed
at the point of contact. Now if the carbon rods are separated a short distance a flaming arc will be drawn out.
The act of making contact between the two carbons is necessary to start the arc and is called STRIKING THE ARC.
When contact first occurs, a high current flows through the carbons. The high current produces heat which vaporizes
the soft center of the positive carbon. This vapor acts as a conductor for the current as the carbons are drawn apart
and will keep the current flowing until the distance between the carbons becomes too great. As the distance between
the carbons increases the resistance of the arc circuit increases.
The carbon vapor concentrates as a small
ball in a depression in the positive carbon. This depression is called the CRATER. This ball of flaming gas in the
crater is intensely luminous and is located at the focus of the reflector. The reflector thus projects the light from
the arc crater into a beam of light, just as the reflector in your automobile projects the light from the lamp filament
into a light beam.
HIGH INTENSITY arcs work on the same principle as low intensity types except that the current
is increased and the diameter of the carbon decreased, to obtain a more brilliant and concentrated source of light.
Figure 141. - The 24-in searchlight.
The high intensity arcs require ventilation systems at all time to prevent overheating.
The arc and reflector
are housed in the DRUM, which is supported on TRUNNION ARMS so that it can be tilted, to elevate or depress the beam.
The BASE supports the trunnion arms so they can rotate. The drum is thus mounted so the light beam can be trained
as well as elevated.
The power supply is brought up to the light through slip rings and brushes inside the
base, thus permitting continual rotation in either direction. In some bases the power is brought up by flexible cables.
These cables must have a long loop left in order to permit rotating the searchlight. This light can be only rotated
one and a half revolutions in either direction.
Trunnion Arm. The trunnion arms support the main
body of the searchlight. The drum is mounted on ball bearings located at the tops of trunnion arms, allowing for free
elevation and depression of the lamp. A locking device is on the trunnion arm to secure the light in elevation and
The trunnion arm rides on ball bearings mounted on the top center of the base, to allow rotation
of the lamp in train. A locking device is provided on the base for securing the light in train.
The drum is the main body of the searchlight and consists of a barrel and access doors. The ventilating motor and
shutter usually are mounted on or in the drum.
Vent Motor. The ventilation motor is located in a hood
mounted either on the top or bottom of the barrel. The motor is connected in parallel with the arc circuit so that
as soon as the lamp switch is closed the motor runs. Notice circuit of wiring in figure 142.
Figure 142. - Wiring circuit for 24-inch searchlight.
Arc Image Screen. The arc image screen is located near the top of the barrel. It contains a lens system
and a ground glass screen. The screen gives the operator an indication of the condition of the arc. A marker on the
ground glass screen indicates the proper position for the positive carbon. The arc image screen does not give a direct
view of the arc, only an image. Figure 143 shows the correct and incorrect positioning of the carbons as viewed in
Figure 143. - Views of carbon position.
Arc Lamp. The lamp consists of the support and carbon feed mechanisms as illustrated in figure 144. The
COLUMN is the upright which supports the two feed heads, and acts as a return for the negative side of the arc current.
The column is hollow and allows air to flow through it to cool the positive head.
Figure 144. - Searchlight lamp mechanism.
Positive-head. The positive head is the mechanism which supports, rotates, and feeds the positive carbon.
These functions are all automatic. The positive head has metal brushes to conduct the current to the carbon.
Obdurator. The obdurate is the metal shield on the positive head. It prevents the heat of the arc from reaching
the gear and other exposed mechanism of the positive head. It also helps direct the light rays back to the reflector.
The lamp has five automatic mechanisms. They are:
Forced draft ventilation.
Rotation of positive carbon.
Negative carbon feed.
FORCED DRAFT VENTILATION
The ventilation motor is usually a motor of approximately 1/12 horsepower connected across the arc circuit. As
soon as the lamp switch is closed the vent motor is energized. In operation of the searchlight, the first thing to
check is the ventilation.
POSITIVE CARBON FEED
The ball of vapor is the main source of light and must be kept at the focal point of the reflector. The focal
point is the point where all light rays from the ball of vapor will be reflected back in parallel lines.
positive carbon burns back, the luminous ball of vapor will move out of the focal point. Therefore, some means must
be made to keep the luminous ball of vapor at the focal point. This is accomplished by feeding the positive carbon
forward as fast as it burns away.
The positive feed system contains a thermostatic switch, a solenoid, a lever
and gear system. The power to rotate the positive head is furnished by the feed motor. The feed motor is connected
across the arc circuit and runs when the lamp switch is closed.
The thermostatic switch in figure 145 is made
of two metal strips welded together. When heated, one strip expands faster than the other and closes the circuit to
the positive mechanism. When the positive carbon is in the proper position the heat rays from the arc vapor do not
fall on the bimetallic strip. The switch will then be open and cuts off the feed current. As the carbon burns back
the heat rays once more strike the bi-metallic strip, closing the feed circuit and feeding the positive carbon forward.
Figure 145. - Thermostatic switch, positive feed mechanism.
ROTATION POSITIVE HEAD
The positive carbon must be rotated all the time in order to keep the crater that holds the ball of luminous gas
symmetrical. The feed motor is connected across the arc. circuit so that the motor runs all the time the switch is
closed. The positive head mechanism is geared directly to the feed motor. The positive head rotates as long as the
light is in operation.
The peep sight is a small circular opening with a colored glass window to give the operator a direct view of the
The reflector is a parabolic mirror which collects the light rays and reflects them in a parallel beam. Two types
are in use, glass and metallic.
Glass Reflector. The glass reflectors are made of optical glass with
the reflecting surface coated with silver. It is efficient but has two disadvantages: first, breakage due to intense
heat in drum and due to shock; and secondly, the. silver peels. This reflector should be cleaned only with alcohol
and precipitated chalk.
Several metals such as chromium steel, coated aluminum and other alloys are used in
manufacturing the METALLIC REFLECTOR. They are not subject to breakage or peeling. This reflector may be cleaned with
bright work polish or any polish not containing an abrasive. In cleaning NEVER USE A CIRCULAR MOTION - always WIPE
FROM the CENTER OUT.
Shutters are provided so the light beam may be secured without securing the lamp. They are mounted in the front
of the barrel and are of two types - the VANE SHUTTER and IRIS SHUTTER.
The IRIS SHUTTER is light proof. It
can be operated manually or by a control motor. This shutter is too slow to be used as a signal shutter. It should
be cleaned and lubricated according to manufacturer's instructions.
The VANE type SHUTTER is used for signaling
and is primarily a speed shutter. May be operated by manual control or by an automatic key. This shutter is not fully
The rheostat has a low resistance and high current carrying capacity. One of its most important functions is to
prevent a direct short circuit when the arc is first struck. It should be set for the proper operating current of
the searchlight, and after its initial setting, should not require adjustment unless a change in the operating current
CURRENT CONTROL NEGATIVE FEED
Feeding of the positive carbon is automatically controlled so that the crater is always at the focus of the reflector.
The negative carbon feed is also automatically controlled, to keep the arc length constant. Two types of automatic
negative feed are used - CURRENT and VOLTAGE types.
A CURRENT CONTROL negative feed system may be identified
by coil wound with heavy strip copper and connected in series with the arc circuit. The current control feed system
depends upon a varying current through the arc to actuate the system. The equipment consists of a current coil and
plunger which regulate a motor drive for feeding the negative electrode. One end of a pivot arm is attached to the
plunger and the other end of the arm is attached to a spring. The current coil is in series in the negative side of
the arc circuit.
When the lamp switch is first closed, a very small current flows. This allows the spring to
overcome the pull of the current coil, and pull the lever arm down so as to engage the high speed forward feed. The
negative carbon is fed forward until it makes contact with the positive carbon. When the carbons strike, there is
a high rush of current. This high current cause the current coil to overcome the pull of the spring and engage the
reverse high speed feed, thus pulling the negative carbon back and drawing out the arc. When the pull of the current
coil and spring tension are in balance the negative carbon will cease to feed.
Any change in arc length after
the arc is struck will be taken care of by the low speed gears. The action of the current coil is the same as in the
high speed feed except the feed lever engages the low rather than the high speed gears.
Figure 146. - Positive carbon feed drive mechanism.
If the negative carbon were fed too far forward the arc current would increase, causing the coil pull to overcome
the pull of the spring tension and engaging the slow speed reverse gears to feed the negative carbon back until the
In a current control negative feed system INCREASING the SPRING TENSION will DECREASE the ARC
LENGTH and decrease of the spring tension will increase the arc length.
VOLTAGE CONTROL NEGATIVE FEED
In the VOLTAGE CONTROL negative feed system, the voltage drop across the arc circuit operates the system. It consists
of a voltage regulator coil, plunger, pivoted arm with a spring attached to the opposite end of the pivot arm. As
soon as the lamp switch is closed there is a high voltage drop across the arc circuit allowing the coil pull to overcome
the spring tension and feed the negative carbon forward. Upon striking the arc there is a high rush of current, causing
a low voltage drop across the arc circuit, pulling the arm down and engaging the high speed reverse feed. The negative
carbon feeds back until the pull of the coil and the spring tension just balance. Any further change due to carbon
burning back will be taken care of by the slow speed units.
The voltage control system can be identified by
looking for a regulator coil. Voltage control will have many turns of fine wire connected in parallel with the arc
circuit. On a voltage control searchlight if the SPRING TENSION is INCREASED the ARC LENGTH INCREASES; if spring TENSION
is DECREASED the ARC LENGTH DECREASES.
Manual control gear is installed on all searchlights so the searchlight may be operated in case of failure of
the automatic system. All of the automatic functions except ventilation can be operated manually. The manual control
gear has two small cranks located on the back of the lamp box; one controls rotation of the positive head, the second
controls the feed of the negative carbon. A push button controls the feed of the positive carbon.
The angle of the negative carbon in relation to the positive carbon must be correct. If the angle is too great
or too small the crater of the positive carbon will burn off on one side and allow the ball of vapor to escape. See
the lamp instructions book for the correct setting and method of adjustment.
Figure 147. - Carbon feed heads for Sperry 36-inch searchlight.
ADJUSTMENT OF CARBON BRUSH PRESSURE
A proper contact between the heads and the carbons is necessary. If brush pressure is too great the carbons will
stick and not feed properly and if too loose will not give good contact. When correctly set it should take a pull
of 8 to 10 ounces to slip the carbons out of the head with the feed rollers raised.
ADJUSTMENT OF POSITIVE CARBON PROJECTION
The positive carbon must project a distance of 9/16-inch for for the 24-inch light, 15/16-inch for 36-inch light,
in front of the positive head. This prevents the noses from burning and assures proper focus. To make this adjustment
allow the searchlight to operate several minutes. Then secure the light and measure the distance from the positive
carbon tip and the positive nose. If this distance is incorrect, the procedure for adjusting follows.
positive carbon is not projecting out far enough shift the lens, figure 145, in the direction you wish the carbon
to feed. Shift lens only about 1/16 inch at a time. After the lens has been shifted the lamp must be operated several
minutes, and rechecked. This must be repeated until the projection is correct.
ADJUSTMENT OF ARC LENGTH
The first step in adjusting the arc length on a current control searchlight is to determine the proper operating
amperage. You can find this in the instruction book on the light. If the switchboard does not have an ammeter in the
arc circuit, connect in a portable ammeter. If the arc current is too high, decrease the spring tension. This will
lengthen the arc and decrease the arc current. If the arc current is too low increase the spring tension. This will
decrease the arc length and increase the arc current.
For adjusting a voltage controlled searchlight feed determine
the proper arc voltage. If the switchboard does not have a voltmeter for the searchlight, connect a voltmeter across
the arc circuit. If the arc voltage is too high, increase the spring tension. If the arc voltage is too low, decrease
the spring tension. Always CONSULT the INSTRUCTION BOOK WHEN MAKING ADJUSTMENTS.
After the preceding adjustments have been made, the lamp should be focused. A focus screw on the back of the lamp
box moves the entire lamp mechanism toward or away from the reflector. Operate the light for several minutes then
secure and measure the positive carbon tip projection. Then measure the distance from the tip of positive carbon to
the center of the reflector. This distance should correspond with the focal length given on the reflector.
In 24-inch searchlights a pair of carbons will last approximately 105 minutes, in a 36-inch light at 150 amperes,
about 90 minutes and at 190 amperes, about 45 minutes. Spare carbons should be stored in original containers in a
dry clean space.
PRECAUTIONS FOR RECARBONING
Always MAKE SURE LAMP SWITCH IS OFF WHEN RENEWING CARBONS.
IF LAMP IS HOT USE GLOVES AND PLIERS to renew
CHECK CARBON to make sure it is not cracked or warped.
Always START an operation WITH COMPLETE
NEW SET OF CARBONS. Never use a set of stubs. After renewing carbons operate the light for about three minutes to
form the crater in the positive carbon.
REAMING POSITIVE AND NEGATIVE NOSES
The positive and negative noses should be reamed when needed to remove carbon and other obstruction to insure
freedom of carbon to feed. Reamers are furnished as spare parts. The following precautions should be followed: In
reaming always hold reamer straight, make sure feed rollers and brushes are lifted, and IF REAMING WHILE LAMP IS HOT.
NEVER ALLOW REAMER TO STOP moving UNTIL NOSE IS COMPLETELY REAMED. This is to prevent reamer from becoming stuck as
the metal cools.
CLEANING REFLECTORS AND DOME
The reflector and dome glass should be cleaned each time after the light is used. NEVER USE POLISH CONTAINING
Use mixture of denatured alcohol and precipitated chalk to clean reflector; the mixture should consist
of three ounces of chalk to 1/2 pint of alcohol. This paste may be removed with cheese cloth. NEVER USE A ROTARY MOTION.
Always POLISH FROM THE CENTER OUT. Use a cloth to remove residue of carbons from the burning interior of drum. This
residue is a white powdery ash. A small paint brush or painter's duster serves as well to remove carbon particles
from the thermostat lens and top of lamp.
The manufacturers instruction book gives information for lubricating base, trunnion arms, and lamp. Grease or
oil should never be used on the heads. To lubricate heads use a mixture of flake graphite and kerosene at least once
a week. The iris shutter should be given a light coat of graphite and kerosene once a month.
PERIODIC TESTS, INSPECTIONS AND CLEANING
AFTER EACH PAIR OF CARBONS -
Wipe reflector, glass door and thermostat lens.
EXTENDED RUN -
Clean interior of drum, thermostat window and lens and the lamp mechanism.
As soon as the reflector has cooled, clean the reflector and front door glass.
After each period of firing of the ship's batteries, inspect all searchlights.
Test local mechanical gear for training and elevating light, then the remote control or
distant mechanical control gear.
Check remote electrical control gear on searchlight.
Operate each lamp long enough to dry out drum and test functioning of lamp mechanism.
light for full pair of carbons.
At sea and during damp weather, energize remote control electrical
Clean reflector and front door glass.
Clean interior of drum, thermostat, window and lens, and lamp mechanism.
nose caps for insulation to heads and to ground.
Examine exposed gears and bearings for lubrication
and for freedom from dirt, sand
Remove, examine and
clean the metal contact blocks which conduct current to the carbons.
Clean carbon feed rollers.
AFTER 50 HOURS OF OPERATION OR QUARTERLY -
After 50 hours of operation and not less than
once each quarter, remove the lamp
from the drum, overhaul, reinstall and reset the lamp.
Inspect, lubricate, and repair equipment in the base.
Chapter 15 Quiz